1. Field of the Invention
The present invention relates to a wireless communication system including plural wireless base stations and a mobile station, a wireless base station that exchanges packet data with another wireless base station, and a wireless communication control method for controlling communication between the wireless base stations and the wireless mobile station.
2. Description of the Related Art
Various types of wireless communication systems have been proposed that include plural wireless base stations each administering service areas that are allocated in a manner such that the service areas partially overlap with one another. In such communication systems, the wireless base stations may be interconnected via a superordinate station and a network so that a wireless mobile station, either in motion or still, that is located within a given service area may establish communication with another wireless mobile station, a terminal apparatus such as a fixed telephone terminal, or a host apparatus that handles large amounts of data. For example, the next generation wireless communication system that is represented by the LTE (Long Term Evolution) enables high speed transmission by arranging data into packets. It is noted that when a mobile station is in communication with a counterpart apparatus while it is in motion, a handover process may have to be performed in order to switch the wireless base station with which the mobile station is currently in communication to a wireless base station located within a destination area toward which the wireless mobile station is moving. Such a handover process may be started when it is detected that the reception quality such as the reception level and reception error rate of the currently established communication has decreased to be less than or equal to a predetermined value. By switching the base station with which communication is established from a current base station to a base station located within a destination area that can provide desired reception quality, the wireless mobile station may continue its communication with the counterpart apparatus via communication with the wireless base station of the destination area.
It is noted that in a wireless communication system for transmitting packet data, generally, packet numbers are assigned to packet data sets of a sequence in order to enable confirmation of packet continuity. For example, Japanese-Translated International Patent Publication No. 2003-523137 (corresponding to PCT Publication WO 01/060016) discloses a technique for enabling packet transmission between GPRS (General Packet Radio Service) and UMTS (Universal Mobile Telecommunication System) and confirmation of packet continuity by embedding sequence numbers in data.
Also, it is noted that in a wireless communication system, plural wireless base stations may be connected via one or more access gateways and a network. For example, 3GPP TR25.912 version 7.0.0 discloses a system in which wireless base stations under an access gateway are interconnected by a logical path, and when a handover process needs to be performed due to movement of a wireless mobile station, data from the access gateway are transferred in RLC-SDU (Radio Link Control-Service Data Unit) via the logical path from a base station that is currently in communication with the wireless mobile station to a destination wireless base station located in the destination area toward which the wireless mobile station is moving. The destination wireless base station temporarily stores the transferred data, and sequentially transmits the stored data to the wireless mobile station when the handover process is completed.
It is noted that the continuity of packet data exchanged between a superordinate apparatus such as an access gateway to a subordinate apparatus such as a wireless mobile station may be determined by monitoring data sequence numbers assigned to PDCP (Packet Data Convergence Protocol) layers and searching for omissions or disruptions in the sequence order of the packet data, for example. Also, packet data continuity may be ensured by implementing retransmission processes in TCP (Transmission Control Protocol) layers, for example.
However, according to the system disclosed in 3GPP TR 25.912 version 7.0.0, packet data continuity may not be completely ensured. Specifically, as can be appreciated from the above descriptions, according to this system, when a wireless mobile station is handed over from an origin wireless base station to a destination wireless base station, packet data are transmitted from the origin wireless base station to the destination wireless base station via a logical path and stored in the destination wireless base station until the handover process is completed. Then, after the handover process is completed, the stored packet data are sequentially transmitted to the wireless mobile station. In this case, although temporary, the continuity of the packet data transmitted from an origin wireless base station to a destination wireless base station may not be ensured. In this system, although continuity of packet data exchanged between a superordinate apparatus and a wireless mobile station may be ensured, packet data transmitted between wireless base stations in RLC-SDU may not be ensured. Therefore, the destination wireless base station may not be able to determine whether packet data transmitted thereto via the logical path correspond to a header packet or whether one or more packets of a packet data sequence have been lost.